Light sources such as light-emitting diodes (LEDs) are an attractive alternative to incandescent and fluorescent light bulbs in illumination devices due to their higher efficiency, smaller form factor, longer lifetime, and enhanced mechanical robustness. However, the high cost of LEDs and associated heat-sinking and thermal-management systems have limited the widespread utilization of LEDs, particularly in broad-area general lighting applications.
The high cost of LED-based lighting systems has several contributors. LEDs are typically encased in a package, and multiple packaged LEDs are used in each lighting system to achieve the desired light intensity. In order to reduce costs, LED manufacturers have developed high-power LEDs that emit relatively higher light intensities by operating at higher currents. While reducing the package count, these LEDs require higher-cost packages to accommodate the higher current levels and to manage the significantly higher resulting heat levels. The higher heat loads and currents, in turn, typically require more expensive thermal-management and heat-sinking systems which also add to the cost (as well as to the size) of the system. Higher operating temperatures may also lead to shorter lifetimes and reduced reliability. Finally, LED efficacy typically decreases with increasing drive current, so operation of LEDs at higher currents generally results in a reduction in efficacy when compared to lower-current operation.
A further problem associated with using fewer high-power LEDs in broad-area lighting—for example, to replace fluorescent lighting systems—is that the light must be expanded from the relatively small area of the die (on the order of 1 mm2) to emit over a relatively large area (on the order of 1 ft2 or larger). Such expansion often results in decreased efficiency, reduced performance, and increased cost. For example, a light panel may be edge-lit and incorporate features that redirect or scatter light. However, it is often difficult to achieve uniform light intensity over the entire emitting area of such panels, with the intensity generally being higher at the edge(s) near the light sources. Also, the emission pattern from such devices is typically Lambertian, resulting in poor utilization of light and relatively high glare.
An alternate approach to producing broad-area lighting is to use a large array of small LEDs positioned over the desired emitting area. Such LEDs may be unpackaged LEDs (i.e., LED dies) or packaged within, e.g., a leadframe and polymeric encapsulation. A large array tends to reduce the cost and efficiency losses associated with optics required to spread out light from a small number of high-power LEDs.
The materials used for the various components of the complete lighting package are often dissimilar and often have different thermal coefficients of expansion (TCEs). This may cause problems during the manufacture of the lighting system, for example, during processing steps involving heating or cooling (e.g., soldering, curing of adhesives and/or encapsulants, etc.), or in the field, either from ambient-induced temperature changes or from self-heating and cooling upon power-cycling of the system.
FIG. 1 shows a system 100 featuring a light-emitter substrate 110, an optical substrate 120, an LED 130, and a region 140. Region 140 may be empty or may include a transparent material or a transparent material in combination with a light-conversion material, such as a phosphor. Light-emitter substrate 110 and optical substrate 120 may be different materials and have different TCEs. When this structure is heated and cooled, stresses will generally develop that may result in cracking of one or both substrates or partial or full delamination of one substrate from the other. Stress-induced changes to the electrical connections to LED 130 can cause intermittent connections or open-circuits or short-circuits.
In view of the foregoing, a need exists for systems and procedures enabling the uniform and economical integration of arrays of low-cost light sources (such as LEDs), phosphors, and optical elements, as well as lighting systems based thereon, which minimize TCE-induced problems.